With increasing interests in problems of health, environment, and food safety, improvements are required of methods for detecting the substances relating to the above problems (chemical substances including substances related to living body (hereinafter referred to as a “target substances”)). For detection of such a target substance, a higher sensitivity of detection is often required. This is because the available amount of the analytes containing the target substance is often very small, and further the target substance is contained in an extremely small amount in the analyte with coexistence of various substances, especially in detection of a protein in a blood. Thus, in the detection of the target substance, high-sensitive detection technique is demanded for detection of an extremely small amount of the target substance in a small amount of an analyte.
To meet the above requirement, methods of measurement are being developed which utilize plasmon resonance with metal particles or a metal structure. On illumination of an incident light in a particular wavelength range onto metal particles or a metal structure, resonance occurs in a limited wavelength range to cause increase of the scattering or absorbance of the incident light. This phenomenon is localized plasmon resonance (hereinafter referred to simply as “plasmon resonance”). The wavelength in which the absorbance becomes maximum is called a plasmon resonance wavelength. At the plasmon resonance wavelength, the transmittance of the incident light through the metal particles or metal structure is decreased significantly. This plasmon resonance wavelength depends on the refractive index of the medium surrounding the metal particles or metal structure. Therefore, the plasmon resonance wavelength can be changed by gathering a target substance from the analyte in the neighbourhood of the metal structure. In other words, the absorption spectrum of the incident light illuminated onto the metal structure can be changed by the target substance gathered in the neighbourhood of the metal structure.
In detection of the substance by utilizing the plasmon resonance with a metal particle or metal structure, usually are detected the shift of the absorption spectrum of the illuminated incident light (i.e., the shift of the resonance wavelength) or change of the absorbance at a specified wavelength. Therefore, a larger shift of the plasmon resonance wavelength or a smaller peak width of the absorption spectrum is desirable for detection at higher sensitivity.
A document, J. Phys. Chem. B, 2004, vol. 108, No. 1, pp. 109-116, describes that the shift of the absorption spectrum depends on the increase of the electric field intensity caused by light irradiation in the neighbourhood of the metal structure: a larger increase of the electric field intensity contributes more the shift of the absorption spectrum.
A document, J. Phys. Chem. B, 2005, vol. 109, No. 8, pp. 3195-3198, describes that a shorter distance between the metal structures causes increase of the electric field strength in the neighbourhood of the metal structures. A document, Optics Communications, 2003, vol. 220, No. 1-3, pp. 137-141, describes that the shorter distance between the metal structures increases the peak width of the absorption spectrum.
A document, J. Phys. Chem. B, 2004, vol. 108, No. 1, pp. 109-116, describes that a random distribution of gold nanoparticles for causing the plasmon resonance makes random the distance between the gold nanoparticles to broaden the resonance conditions depending on the distance between the gold particles to lower the detection sensitivity, since the conditions of the plasmon resonance depend on the distance between the gold particles.
According to the above documents, J. Phys. Chem. B, 2005, vol. 109, No. 8, pp. 3195-3198 and Optics Communications, 2003, vol. 220, No. 1-3, pp. 137-141, the smaller intervals between the metal structures can increase the electric field strength between the metal structures and enables increase of the shift of the plasmon resonance wavelength, but can increase the peak width of the absorption spectrum. Thus, the increase of the shift of the plasmon resonance wavelength and decrease of the peak width of the absorption spectrum cannot be achieved simultaneously. Therefore, it is not easy to provide a detection apparatus for detecting a target substance with a high sensitivity.
To solve the above problems, the present invention intends to provide an apparatus for detecting a target substance at a high sensitivity, and a method for detection employing the detecting apparatus.